Gene product of the Major Histocompatibility Complex (MHC) present foreign antigen to T-lymphocytes and are thus essential components of the immune system. MHC genes are the most polymorphic loci known for vertebrates and it is generally assumed that pathogen diversity and the rapid rate of pathogen evolution are the factor responsible for the maintenance of this unprecedented genetic diversity. It is also thought that specific hypermutational mechanisms have evolved that enhance the genetic diversification of the MHC. However, the general failure to find susceptibility to specific diseases associated with specific MHC alleles has cast some confusion over this general view. Many of the mechanisms that could either generate or maintain MHC genetic diversity would produce distinctive patterns of sequence diversification. Thus, characterizing patterns of sequence diversification is a powerful approach to understanding the origin, maintenance, and functional significance of MHC polymorphisms. This is the only kind of approach that can provide a broad evolutionary view of host- pathogen interactions and coevolution. We propose to characterize the pattern of sequence diversification in class II MHC genes from our collection of 115 wild-mouse derived haplotypes representing 10 species and subspecies of the genus Mus. High resolution restriction mapping will be used to assess sequence diversification patterns over the region encompassing the A alpha, A beta, and E beta class II genes. Exons (and some introns) of these genes will be sequenced using polymerase chain reaction (PCR) methodology. These data will allow the construction of phylogenetic trees of allelic lineages and will provide answers to the following questions: 1) What are the minimum number of founding alleles that could account for current allelic lineages observed in each of the species? 2) Are allelic lineages "conserved" or distributed randomly across different Mus species? 3) Is the diversification of antigen binding sites random or patterned? 4) Are class II genes experiencing positive selection for sequence diversification? 5) What is the relative contribution of hypermutational events such a segmental exchange to sequence diversification? 6) Are these segmental exchange events site specific? 7) Does subunit assembly of class II heterodimeric molecules constrain diversification? 8) what sequences control chromatin structure in A beta lineages and are these sequences evolutionarily conserved? Answers to these questions are fundamental to understanding the evolution of host- pathogen interactions and the functional significance of MHC polymorphisms.